The present invention relates to a position control system in a D.C. motor.
In order to precisely control the position of a D.C. motor, the D.C. motor is generally controlled in two modes. The first mode is a speed detection mode wherein the rotating speed of the motor is controlled so that the motor rotates to a desired position at a fast speed. The second mode is a position detection mode, which is conducted after the completion of the speed detection mode. In the position detection mode, the motor position is precisely controlled so that the motor stops at the desired position.
In the conventional system, the motor speed control in the first mode is conducted by the frequency control method. The frequency control method inevitably produces a steady-state deviation between the instructed rotating speed and the actual rotating speed of the D.C. motor. The steady-state deviation varies depending on the load connected to the D.C. motor. Thus, the conventional system can not ensure a stable operation when the load connected to the D.C. motor changes.
Accordingly, an object of the present invention is to provide a D.C. motor position control system which ensures stable operation of the motor even when the load connected to the D.C. motor varies.
Another object of the present invention is to provide a D.C. motor position control system which precisely controls the position of the D.C. motor and stops the D.C. motor at a desired position.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
To achieve the above objects, pursuant to an embodiment of the present invention, the D.C. motor rotation is controlled by the speed detection mode, and, then, the position detection mode. In the speed detection mode, phase-locked-loop (PLL) control is employed, wherein the phase of a motor position indicating signal and the phase of the speed instructing signal are compared so as to control the rotating speed of the D.C. motor. The phase-locked-loop (PLL) control eliminates any the steady-state deviation of the actual rotating speed with respect to the instructed speed. Therefore, an accurate speed control is achieved even when the load connected to the D.C. motor varies.